The usual frequency range of a radar filling level meter that functions on the basis of microwaves is between approx. 1 GHz and 100 GHz. Radar meters determine the filling level of a filling material in a container from the transit time of the microwave measurement signals. Transit time methods use the physical principle that the path of the microwave measurement signals is equal to the product of the transit time and the propagation velocity of the measurement signals. In the case of level measurement, the path is equal to double the distance between the antenna and the surface of the filling material. The useful echo signal, i.e., the portion of the measurement signals that is reflected by the surface of the filling material, and its transit time are determined using the so-called echo function or the digital envelope curve. The envelope curve depicts the amplitudes of the echo signals as a function of the distance between the antenna and the surface of the filling material. The filling level itself can then be determined from the difference between the known distance from the antenna to the bottom of the container and the distance determined by the measurement from the surface of the filling material to the antenna.
Radar meters are differentiated between meters which function according to the pulse radar principle and use broadband high-frequency pulses, and FMCW (frequency-modulated continuous wave) meters, in which the frequency of the continuous microwaves of a wavelength λ is periodically linearly modulated using, for example, a sawtooth voltage.
To ensure consistently good measurement performance, the antenna elements emitting and receiving the measurement signals and the measurement electronics are protected on the process side against environmental influences using process separation elements. This protection is very important, since the filling level meters are exposed to high temperatures, high pressures, and/or aggressive chemical media, depending upon the place of operation. Depending upon the conditions prevailing at the measuring site, the requirements laid down for the protection of the sensitive electronic components are correspondingly high.
From published international patent application, WO 2006/120124 A1, a filling level meter is known, in which a horn antenna is filled at least partially with a temperature-stable dielectric material. The dielectric material is dimensioned in such a way that, at normal temperature, a defined distance between the outer surface of the dielectric filling material and the inner surface of the adjacent antenna element exists. Owing to the design, the filling material can expand with rising temperatures so that no mechanical stresses occur within the filling material.
In the radar meters that Endress+Hauser sells under the designation MICROPILOT®, the process separation device is usually made of PTFE. PTFE has the advantage that it is almost transparent to the microwaves. Furthermore, it shows sufficient temperature, pressure, and/or chemical stability for a variety of applications in process automation. In addition, PTFE has the advantage that it impedes the formation of deposits. If deposits are still formed on the process separation device, problem-free cleaning is possible.
Above 400° C., plastics are no longer stable. For this reason, process separation elements made of ceramics are preferably used in high-temperature and high-pressure ranges. In comparison to plastics, ceramics do, however, have the disadvantage that they have relatively high dielectric constants. This significantly reduces the high-frequency-compatible dimensioning of a process separation device. The smaller the dimensions of the process separation device are, the more prone they are to the formation of condensate and deposits. If, however, the dimensions of the process separation device are adapted to the condensate and deposit problems, increasingly higher modes are excited. If higher modes are excited, the ringing, which describes the portion of the undesired reflections in the antenna region, is amplified. The more intense the “ringing” is, the worse the measurement performance of the filling level meter becomes.
From German patent, DE 102010031276 A1, a filling level meter is known that is suitable for the high-temperature and the high-pressure range. Here, too, a process separation element that allows the microwaves to pass largely undisturbed is inserted into a wave guide in which the high-frequency measurement signals are transmitted. In order to avoid the aforementioned formation of deposits and/or condensate, the process separation element is designed as a hollow piece, wherein the end region facing away from the process is adapted to the diameter of the wave guide, and the end region facing the process is preferably designed to be cone-shaped or pyramid-shaped. The wall thickness of the process separation element in the emission area is approximately half the wavelength of the measurement signals sent and received. In this way, undesired reflections of the measurement signals are largely avoided. The process separation device is produced of a plastic material, a ceramic material, or a dielectric composite material. Preferably, ceramic material is used, since it is highly pressure and temperature resistant.
With the known design, there is, however, the risk that the apex of the cone or pyramid breaks as a result of external mechanical forces—e.g., as the result of a blow. If the process separation device is not sealed, it can no longer fulfill its protective function.